It is generally better to not do this - the experts go further and say when you receive a date/time value you should split it into separate date and time fields. This is because of the way QV stores the in-memory data.

Of course, if you need to do this as a part of your load to perform some sort of caclulation, and the date and time are normal QV date and time values, then you can simply add them together to get a date/time value:

TimeStamp(myDate + myTime, 'YYYY/MM/DD hh:mm:ss')

(Time stamp allows you to decide on the display format and has no effect on the underlying value)